Diagnostic imaging is an important tool of medical and veterinary healthcare today. In particular, X-ray imaging is routinely employed for visualizing different (normal, affected or malignant) tissues in the human and animal body. Furthermore, diagnostic imaging is also employed for imaging other types of material than whole animal bodies, including in vitro tissue imaging and non-biological imaging. An example of the latter is security control imaging.
X-rays penetrate the investigated material and become absorbed mainly by photoelectric effect and Compton scattering due to the atoms in the material. The absorption amount of the X-ray depends on the attenuation coefficient of the given material, which in turn depends on its atomic weight and density. Traditional absorption or attenuation based X-ray imaging, thus, acquires a two-dimensional (2D) image of the X-ray intensity differences in the material. Such attenuation based X-ray imaging works rather well for several medical applications though has limitations in a relative low contrast, sharpness, resolution and image quality in soft tissue regions of medical interest.
Improvements have and are continuously developed in the field of X-ray imaging. The problem of low contrast was solved in the early 1970's by one of the most significant breakthroughs in diagnostic imaging, namely X-ray computed tomography. In X-ray CT, several hundreds of X-ray images are generally taken of a patient from different views 360° around the patient body. These multiple images are co-processed in order to provide a high contrast three-dimensional (3D) representation of a portion of the patient body.
In the very beginning of CT imaging, the X-ray gantry was static with a rotating object, though further developments was rapidly directed towards a static object by new X-ray CT generations with a rotating X-ray source and detector or only one or more rotating X-ray sources. Improvements relating to CT imaging have also taken place, among others, in the design of the radiation gantry, allowing ever faster rotations and image acquisitions from the different rotational views. Also the X-ray source and detector techniques for CT imaging have been improved highly since the first introduction of X-ray CT diagnosis.
CT imaging is though marred by several drawbacks, in particular due to the need for rotating the gantry around the object to be imaged. This, above, all limits the availability of X-ray sources that can be fit into a rotating gantry and can be designed to cope with the high rotational speeds of CT gantries (today about three revolutions per second). X-ray CT imaging also has too low contrast and geometrical and temporal resolution for several medical applications, including heart imaging and angiography.